The cleaning of metal parts in industrial finishing is frequently effected in spray-washing plants in which an aqueous washing fluid is circulated and sprayed by means of nozzles onto the metal parts to be cleaned.
Washing fluids are frequently used which contain organic anti-corrosive components, particularly alkanol amines and fatty acids and/or aromatic carboxylic acids. Furthermore, the solutions may contain small quantities of inorganic components such as sodium nitrite or phosphates as inhibitors for example, for non-ferrous metals, dissolving intermediaries and preservatives. The pH value of the solutions used generally lies in the range of from 7.5 to 11.
In addition to the components already mentioned, these cleaning solutions may contain surface-active compounds or tensides for the purpose of obtaining a degreasing and cleaning action. Since the cleaning solutions must not foam when used in spray-washing plants, nonionic tensides or combinations of non-ionic tensides are customarily used.
The reason for the use of nonionic tensides is their low solubility at higher temperatures (inverse solubility) and thus the precipitation of a phase, richer in wetting agent and finely distributed, at the so-called turbidity point. Nonionic tensides exhibit a distinctly lower tendency to foam above the turbidity point. Nonionic surfactants customarily used are products of addition of ethylene oxide and/or propylene oxide to compounds having a labile hydrogen atom such as alkylphenols, fatty alcohols, fatty amines, fatty acids, fatty acid amides or similar compounds, as well as block polymers of polyethylene oxide and polypropylene oxide.
When used for a long period of time, these known cleaning solutions tend to precipitate the wetting agents, present at the turbidity point in the form of a fine dispersion in large drops and layers on the metal surface, whereby the tensides on the metal surface are passed to the rinsing step where they are flushed out and are no longer available for the cleaning method and for regulating the foam. The risk of the precipitation of wetting agent is particularly great in washing plants having a high pump performance and relatively small alkaline solution containers. The precipitation of coarsely distributed tensides on the metal surfaces causes, on the one hand, the disadvantage of a reduction in the efficacy of the washing action which is substantially based on the tenside content of the cleaning solution and, on the other hand, the disadvantage that the components remaining in the solution, such as the organic anti-corrosive components, can lead to the foaming-over of the washing plants after precipitation of those nonionic tensides which have a foam-inhibiting effect when finely distributed.